4 fast cores, 4 slow cores
CPU's, just like car engines, are all about trade-offs. The two biggest ones are performance and efficiency. If you have a crazy 900hp v12 in your car you won't be driving very far before you have to fill up again. Same thing with CPU's, a super high-performance processor will mean your battery won't last very long.
On the other hand having a super efficient CPU that sips energy will give you great battery life but you might go crazy with how slow your phone will be.
Traditionally you got 4 cores somewhere in between those two options. A compromise of performance and efficiency. Now, with these 8 core processors for example, you get 4 high-performance cores and 4 slow but high-efficiency cores. That way when you're playing a mobile game or something you get the performance you want from the 4 fast cores. If you're just send text messages every now and then your phone switches to the slower cores to give you longer battery life.
It is known as Big.Little. (Source)
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